High-pressure oxygen-17 NMR study of the dihydroxo-bridged rhodium(III) hydrolytic dimer. Mechanistic evidence for limiting dissociative water exchange pathways

Rate constants and activation parameters for water exchange on the Rh(III) hydrolytic dimer [(H2O)(4)Rh(mu-OH)(2)Rh-(H2O)(4)](4+) were determined using O-17 NMR spectroscopy as a function of temperature (308-323 K) and pressure (0.1-150 MPa). This represents the first variable-pressure O-17 NMR study conducted on a polynuclear metal aqua ion. Two pathways for water exchange with similar rates were found for H2O coordinated at positions cis (k(cis)) and trans (k(trans)) to the bridging OH groups on the Rh(III) centers. The bridging OH groups were not found to exchange with the bulk solvent, indicating that they are substitution inert on the time scale of these experiments. The kinetic parameters for water exchange on the fully protonated Rh(III) dimer at [H+] = 1.0 M and mu = 2.0 M are as follows: k(trans)(298) = 8.5 x 10(-7) s(-1), Delta H*(trans) = 150 +/- 2 kJ mol(-1), Delta S*(trans) = +141 +/- 7 J K-1 mol(-1), Delta V*(trans) = +8.5 +/- 0.8 cm(3) mol(-1) (+8.5 +/- 0.6 cm(3) mol(-1) at [H+] = 2.0 M and mu = 3.0 M); k(cis)(298) = 5.4 x 10(-7) s(-1), Delta H*(cis) = 159 +/- 8 kJ mol(-1); Delta S*(cis) = +168 +/- 24 J K-1 mol(-1), Delta V*(cis) = +10.1 +/- 0.3 cm(3) mol(-1) (+10.9 +/- 1.0 cm(3) mol(-1) at [H+] = 2.0 M and mu = 3.0 M). In comparison to the water exchange for [Rh(H2O)(6)](3+), the introduction of bridging OH groups in the dimer was found to labilize the bound waters but not to the same extent as deprotonation of the monomer. Assuming an expansion of the ground-state partial molar volume of 3-5 cm(3) mol(-1) due to the labilizing effect of bridging OH groups in the dimer, the kinetic results suggest a limiting dissociative pathway, D, for water exchange in both the cis and trans positions. It is postulated that dissociation of a more labile trans water molecule in the Rh(III) dimer will lead to a five-coordinate intermediate which can undergo a rapid Berry pseudorotation to account for the similarity in rate and activation parameters for exchange of both cis and trans water molecules.